The invention relates to a method and apparatus for the oxidation of reactants in an aqueous reaction medium using gaseous molecular oxygen to increase the concentration of oxygen in the aqueous reaction medium and improve the efficiency of the reaction process.
Industrially, aqueous phase oxidations are carried out using a variety of oxygen sources such as air, oxygen and oxidative reagents such as hydrogen peroxide. At industrial scales, oxidations with air and oxygen are a far lower cost alternative compared to oxidation with oxidative reagents but pose a challenge due to their inherently low solubility of oxygen in water. Oxygen solubility in water decreases with increase in concentration of solutes, especially ions, as well as with an increase in temperature. When oxidation reactions are utilized in the production of chemicals, waste water and scrubber effluents often have high concentrations of solutes and the oxidation is carried out at an elevated reaction temperature Both of these factors decrease the effective oxygen solubility
Air is often perceived as the lower cost alternative compared to oxygen for gas-liquid oxidation systems but at times, oxidation with air is not intense enough in a given apparatus, oxidation system or gas-liquid contacting equipment and oxygen provides a viable alternative.
There are a variety of oxidation reactors used in process industry today and in addition to dissolution of oxygen, many other process requirements such as heat transfer, suspension of solids, mixing and safety, including keeping the vapor space outside of explosive limits, influence the selection of the type of reactor. Economic factors such as equipments costs, power consumption, operational complexity and reliability and maintenance are also important in determining an optimum and viable oxidation system.
One key consideration in the design of any oxidation system using gaseous molecular oxygen is to ensure the optimal utilization of oxygen.
Typical oxidation reactors are stirred tank reactors or columns under ambient pressures where oxygen is sparged at the bottom. In a simple bubble column or tank, where oxygen is sparged, gas bubbles rise in the aqueous medium while some oxygen gas dissolves in the aqueous medium and the remaining oxygen disengages from the liquid pool when it reaches the liquid surface at the top. If the tank or column is open which is typical of most mineral processing and waste water oxidation systems, oxygen disengaging the liquid surface, along with vapors of aqueous medium escape to the atmosphere.
For the production of chemicals, however, it is not often an option to allow an oxygen rich stream to escape to the atmosphere and the process industry uses tanks and columns with lids. The disengaging gaseous bubbles are collected in the ullage of the tank or vapor space of the column and recycled back to the sparger by means of a compressor or blower. This can add significant amount of costs in terms of energy and processing equipment despite a more effective utilization of oxygen.
Stirred tank reactors (STR) with gas sparging often provide better dissolution of oxygen compared to simple bubble columns or non-agitated tanks. However, the use of STR is limited to applications with smaller oxidation volumes and scale up to very large reactors is not common. In addition, for better utilization efficiency, an oxygen recycle loop may be necessary.
High pressure bubble columns and STR often provide far superior performance with oxidation and gas dissolution but they are several fold higher in cost compared to ambient pressure systems and in addition may require an oxygen recycle loop. When the rate of the oxidation reaction is slow, high pressure systems assist in intensifying the reaction due to a higher dissolved oxygen concentration. The use of high pressure bubble columns and STR are generally limited to applications with smaller oxidation requirements.
For larger oxidation volumes, oxidation systems of large tanks with external loops are often used. Oxygen in these processes is dissolved in a small stream of an aqueous medium withdrawn from the main vessel and oxygen gas is intensely mixed using static or dynamic mixing devices, sometimes even saturated and reintroduced along with very large number of gas bubbles into the main vessel. External pumps are used to drive the fluid through the external loops. In some systems, the aqueous medium withdrawn is subjected to oxygen at higher pressure in a separate vessel and oxygen is dissolved and saturated at higher pressure prior to being reintroduced back into the bulk liquid to form bubbles. For all these systems, the intention is to maximize either oxygen utilization or the rate of oxygen uptake.
Special consideration can be given to oxidation reactions where “M” is dissolved in an aqueous medium and oxidized with molecular oxygen. This is represented by the following reaction:M+x/2O2→MOx 
In a given oxidation system, a reaction is classified as very slow if the dissolution of oxygen is much faster compared to its consumption in the oxidation reaction.
In such a case, there will be a finite concentration of dissolved oxygen in the bulk of the aqueous phase. The concentration of the dissolved oxygen will be between negligible on the lower limit value and the equilibrium solubility concentration as the upper limit. The specific rate of oxidation is mathematically expressed as:R=kmn*[M]m*[O2]n Where R is specific reaction rate;kmn is the oxidation rate constant, generally a function of temperature;mth order with respect to M;nth order with respect to oxygen;[M] is the concentration of solute to be oxidized; and[O2] is the concentration of dissolved oxygen.
The concentration of solute to be oxidized [M] in the batch reactor starts at a very high concentration at the beginning of a batch operation and as oxidation proceeds, [M] reaches lower concentration slowing the specific reaction rate considerably towards completion of the batch.
The specific rate is maximized when the concentration of dissolved oxygen approaches solubility at a given pressure. In order to maintain specific rates close to the maximum, high levels of dissolved oxygen are necessary. These high levels are achieved by contacting a large excess of molecular oxygen gas which must be reutilized by recycling or wasted and both of these add to capital costs or operating costs.
In addition to maximizing the specific oxidation rate, reactions are conducted at higher temperature. Generally the increase in the reaction temperature increases the kinetic rate constant “kmn” but reduces the solubility of oxygen and dissolved oxygen concentration. Alternatively increasing pressure along with temperature is a different approach but in very large scale production, the oxidation of low cost feed stocks such as minerals, ores and low cost inorganic chemicals, capital intensive large pressure vessels are not economically attractive and therefore a lower cost, efficient solution is necessary. The present invention addresses this need by providing an improved oxidation process and apparatus that not only achieves high oxygen utilization efficiency, but also offers enhanced rates of oxygen uptake.